Improve: run the test suite in parallel

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
       - run: uv run ruff format --check .
       - run: uv run ruff check .
       - run: uv run mypy
-      - run: uv run pytest
+      - run: uv run pytest -n auto
 
   workflows:
     runs-on: ubuntu-24.04

.pre-commit-config.yaml

@@ -19,7 +19,7 @@ repos:
         types: [python]
       - id: pytest
         name: pytest
-        entry: uv run pytest
+        entry: uv run pytest -n auto
         language: system
         pass_filenames: false
         always_run: true

CONTRIBUTING.md

@@ -73,7 +73,7 @@ Run each with `uv run python tools/<script>`.
 CI runs the same checks:
 
 ```sh
-uv run ruff format . && uv run ruff check . && uv run mypy && uv run pytest
+uv run ruff format . && uv run ruff check . && uv run mypy && uv run pytest -n auto
 ```
 
 The tests need no API keys and no internet connection. Ruff limits each function's complexity, branches and arguments. Split a new function that goes over a limit instead of adding a `noqa`. `pytest` measures coverage and fails below 85% or on any warning. CI also lints the workflows with actionlint and zizmor, and scans the image with Grype; a fixable critical vulnerability fails the build.

pyproject.toml

@@ -26,6 +26,7 @@ dev = [
     "mypy>=1.18",
     "pytest>=8.4",
     "pytest-cov>=7",
+    "pytest-xdist>=3.8.0",
     "resvg-py>=0.2",
     "ruff>=0.14",
 ]